1. Field of the Invention
The invention relates to a direct current (DC)/DC converter.
2. Description of the Related Art
Various home appliances including mainly TV sets and refrigerators, or various electronic devices including mainly laptop computers, mobile phone terminals and Personal Digital Assistants (PDAs) operate by receiving external power, and batteries therein can be charged by power from an external power supply. In addition, in the home appliances and the electronic devices (generically referred to as electronic devices hereinafter), a power supply apparatus for AC/DC (alternating current/direct current) conversion of a commercial AC voltage is disposed, or the power supply apparatus is disposed in an external power supply adaptor (AC adaptor) of the electronic devices.
The power supply apparatus includes a rectifier circuit (a diode bridge circuit) for rectifying an AC voltage, and an insulated DC/DC converter for reducing the rectified voltage and supplying the reduced voltage to a load.
FIG. 1 shows a structure of a DC/DC converter 100r studied by the inventors of the invention. A specific structure of the DC/DC converter 100r cannot be considered as an ordinary technology well known to those skilled in the art.
A DC input voltage VIN from a rectifier circuit (not shown in the drawings) disposed in a front part of the DC/DC converter 100r is input to an input terminal P1 of the DC/DC converter 100r. The DC/DC converter 100r reduces the input voltage VIN and supplies the reduced input voltage VIN to a load connected to an output terminal POUT.
The DC/DC converter 100r mainly includes a switch transistor M1, a transformer T1, a first diode D1, a first output capacitor Co1, a control circuit 10r, and a feedback circuit 20r. In the DC/DC converter 100r, a primary side region and a secondary side region of the transformer T1 must be electrically insulated. The feedback circuit 20r includes resistors R1 and R2 for dividing an output voltage VOUT, a shunt regulator 22, and an optical coupler 24.
The shunt regulator 22 is an error amplifier for amplifying an error between a divided output voltage VOUT′ and a reference voltage VREF corresponding to a target value of the output voltage VOUT. The optical coupler 24 feeds a feedback signal corresponding to the error between the output voltage VOUT and the target voltage back to the control circuit 10r. The control circuit 10r controls a duty ratio of the on/off of the switch transistor M1 by using pulse modulation, so that the output voltage VOUT is made to be in consistent with the target value.
The control circuit 10r can operate at a power supply voltage VCC of about 10 V, and if the input voltage VIN (about 140 V) is used to drive the control circuit 10r, the efficiency is deteriorated. In addition, the voltage VOUT reduced by the DC/DC converter 100r is generated on the secondary side of the transformer T1, and thus the voltage VOUT cannot be supplied to the control circuit 10r disposed on the primary side.
Therefore, an auxiliary coil L3 is disposed on the primary side of the transformer T1. The auxiliary coil L3, the second diode D2, and the second output capacitor Co2 function as an auxiliary DC/DC converter for generating the power supply voltage VCC for the control circuit 10r. In the DC/DC converter 100r, the power supply voltage VCC is proportional to the output voltage VOUT, and a proportionality factor is dependent on a ratio of number of turns of the secondary coil L2 to the auxiliary coil L3 of the transformer T1.VCC=VOUT×ND/NS 
Wherein, NS is the number of turns of the secondary coil L2, and ND is the number of turns of the auxiliary coil L3.    Patent Document 1: Japanese Patent Publication No. 1997-098571    Patent Document 2: Japanese Patent Publication No. 1990-211055